1. Field of the Invention
The present invention relates to a method of manufacturing glass yarns, and, more particularly, to a method of manufacturing a plurality of glass yarns at one time from a plurality of strands formed by separately gathering a large number of glass filaments drawn from a multi-hole bushing.
2. Description of the Prior Art
In manufacturing glass fiber, molten glass is drawn at high speed through a large number of orifices formed on the bottom of a bushing so as to form a large number of filaments. After sizing agents are applied to the filaments, they are gathered into a strand by being passed through a gathering roller. The strand is then wound into a cake by a winder while being traversed by a traverse motion. The glass fiber which has been formed into the cake is utilized mainly in the forms of glass roving or chopped strand and glass yarn. To manufacture glass roving, the cake is dried by applying heat for a predetermined period of time. The winding tube is then pulled out from the cake, and the strand is drawn out from inside the cake (this being generally called the internal drawing method). Subsequently, a predetermined number (generally, 10 to 30) of such strands are gathered in parallel so as to form a roving, and the thus-formed roving is wound by a winder. The chopped strand is formed by cutting the roving with a cutter. The thus-formed glass roving or chopped strand is used as a reinforcing material for a fiber glass reinforced plastic or a fiber glass reinforced thermoplastic. In manufacturing a glass yarn, the cake is air dried for a predetermined period of time, and is rewound by drawing out the strand from outside the cake (this being generally called the external drawing method) by means of rewinder. The drawn strand is twisted to form a glass yarn. The thus-formed glass yarn is wound on a bobbin so that it can be used as a material for glass fabric.
The efficiency of producing glass fiber is determined by the weight of glass fiber that can be drawn from a bushing per unit of time (generally this is referred to as one spindle amount), and attempts have therefore been made to increase this one spindle amount by providing a multi-hole bushing. The multi-hole bushing produces a thick strand, and the thick strand raises problems in terms of resin impregnation or surface smoothness when it is used to form a roving which is used as a fiber reinforcing material. These problems, however, can be solved by the division of one strand into several smaller strands and the winding of these smaller strands together on one winding tube. More specifically, in manufacturing a glass roving, a strand is dried in a state wherein it is divided into a plurality of smaller strands, and is drawn out from inside the cake as a bundle of smaller strands to be supplied to a roving winder or cutter, ensuring that solutions can be found to the problems of resin impregnation and surface smoothness of a product which may arise when the roving is used as a material of fiber reinforced plastic or fiber reinforced thermoplastic.
On the other hand, the strand which is used to form a glass yarn is formed by filaments each of which generally has a diameter of 5.mu. to 10.mu.. Sizing agents which are mainly composed of a starch type substance are coated on the filaments to gather them into a strand, and the cake formed is air dried. As a result, the resulting strand ehhibits poor integrity when compared with a strand which is used to form a roving, and therefore readily becomes fluffy. Further, since the strand which is used to form a glass yarn must be rewound and drawn out from outside the cake by a rewinder while the cake is being turned around, the strand tends to get even more fluffy, so the conditions of the rewinding process have a great influence on the quality of a product. Applications of glass yarn include fields which demand high quality, such as use as glass cloth for printed circuit boards. Hence it is known to those skilled in the art that methods that can be used to form a glass roving cannot be applied to the manffacture of a glass yarn as they are.
In manufacturing a glass yarn, one way of increasing the spindle amount is to increase the filament drawing speed. However, there is a limit to the extent to which drawing speed can be increased and it is impossible to double it so as to be able to double the production amount owing to the occurrence of filament breakage. The other way consists in the method of forming divided strands with a multi-hole bushing, as in the case of the manufacture of a roving. In this method, divided strands are wound on a plurality of winding tubes aligned in the longitudinal direction of one collet. For example, if a bushing having 600 holes is used, the filaments formed are divided into three blocks to form three strands with each block consisting of 200 filaments, and the three strands are respectively wound on the three winding tubes mounted separately on one collet. This method therefore produces three cakes on three individual winding tubes with each cake formed from one strand.
The above-described known method of manufacturing glass yarns using a multi-hole bushing uses a large number of winding tubes longitudinally aligned, and therefore requires a collet which is as long as one meter or more. In consequence, this method is not safe in high speed operations and involves a troublesome operation of pulling cakes out from the collet. Further, since the relative positional relationship between the collet and the bushing varies greatly at the two ends of the collet, strand quality is adversely affected. As a result, the the number of strands that can be wound in this way is limited to three, and no significant increase in productivity can be expected.
Accordingly, it has been proposed to wind a plurality of strands on one winding tube as one cake, as in the manufacture of a glass roving, to rewind the plurality of strands from the cake, and to twist each of the rewound strands and wind it on a bobbin. However, this propowal has not been put into practical use because of problems involving the generation of fluffs during rewinding and the occurrence of filament breakage, as well as the difficulty of pulling out a strand end which is attributable to the winding of a plurality of strands into one cake.